This invention relates to the field of photodetectors, and, in particular, to traveling-wave photodetectors which eliminate backward wave effects, such traveling-wave photodetectors being particularly useful in modem communications systems.
Optical communication links, using either fiber or free-space as the communication medium, allow the transmission of large amounts of information over long distances. Of considerable importance is the quality of the received signal, as measured by the system""s Signal-to-Noise Ratio (SNR). One way to achieve a high SNR for a conventional Intensity Modulation/Direct Detection (IMDD) link is to increase the transmitted power. Because the signal power increases quadratically with increasing optical power, while the shot noise of the detector increases only linearly, increasing the transmitter power will always increase the SNR.
One limitation to the ultimate SNR one can achieve is set by the power-handling capability of the receiving photodetector. Achieving a high power-handling capability at low communication frequencies is relatively easy: one just makes the photodetector surface larger. At microwave frequencies, however, this is problematic because a larger area means a larger capacitance, and hence a reduction in the bandwidth over which the signal can be received.
One solution to this problem is to fabricate traveling-wave photodetectors. The traveling wave structure allows one to achieve the large dissipation area needed for high power operation, while eliminating the effect of increased capacitance. However, traveling wave photodetectors also generate backward waves as well as forward waves, and this backward wave can cause either a degradation in the frequency response or, if properly terminated at the input, no frequency degradation, but a 6 dB loss in response across the entire band.
High-frequency p-i-n photodiodes are generally known. A high-power traveling-wave p-i-n photodetector using a conventional forward-backward wave structure has been fabricated and tested by a group at Sandia National Laboratories, as described in the V. M. Hietala et. al. article entitled xe2x80x9cTraveling-wave photodetectors for high-power, large bandwidth applicationsxe2x80x9d, IEEE Transactions on Microwave Theory and Techniques, Vol. 43, pp. 2291-2298 (1995). This device was limited in its frequency response by trapping effects in the absorbing region and not by backward wave effects. Nevertheless, this device does presently represent the state of the art for high-power traveling wave photodetectors.
A schematic block diagram of a conventional traveling wave photodetector is shown in FIG. 1. Lightwave 10 traveling to photodetector 12 is gradually converted along the length of the device into photocurrent for load 18, such as a terminating load with the characteristic impedance ZO of the line. Photodetector 12 has an inner region 12a consisting of an optically absorbing material which is often located between two non-absorbing materials. For a p-i-n planar structure, which is often used for waveguide detectors, these materials would be the p-type and n-type layers, between which the intrinsic absorbing layer is sandwiched. The outer portion 12b consists of the metal electrodes that form the transmission line structure. The doping of the p and n-type layers is such that, at the metal-semiconductor interface, an ohmic contact is formed. This allows one to reverse-bias the p-i-n structure, so that the photo-generated charge can be swept out of the depleted intrinsic layer, into the conductive n and p layers, and subsequently into the metal electrodes. Each conversion 13 in every element dx generates both forward current wave 14 and backward current wave 16. The forward waves sum constructively if the velocity of the current (RF) wave and the optical wave are the same, a condition known as xe2x80x9cvelocity-matchingxe2x80x9d. The backward wave, however, sums destructively, with the result that for sufficiently high signal frequencies or sufficiently long detectors, that part of the signal generated by the reflected backward wave is lost. This is shown in FIG. 2, wherein the response of the detector at higher frequencies is flat, but reduced by 6 dB. This variation in response can be removed by terminating the input end of the photodetector with the characteristic impedance ZO of the line. The backward wave is then absorbed in this impedance, and does not reflect back into the forward wave. The frequency response is then flat across the entire frequency range. The penalty in doing this, however, is that the response is reduced everywhere by 6 dB, and not just at the higher frequencies.
When using the conventional approach a choice must then be made between a flat frequency response and a 6 dB reduction in signal strength, or a reduced bandwidth at full signal strength. This shortcoming is due entirely to the backward-wave effect.
Therefore, there exists a need for an effective traveling-wave photodetector which eliminates backward wave effects. The present invention provides a unique solution to meet such need.
In accordance with the present invention, a traveling wave photodetector of practically unlimited length, that has no roll-off at higher frequencies or loss in response due to backward-wave effects, can be fabricated. The elimination of backward-wave effects is achieved by coherently summing the outputs of a large number of discrete waveguide photodetectors, with each of these discrete photodetectors being sufficiently short so as to avoid, in that particular photodetector, any backward wave effects. Summation can be achieved by a variety of techniques, all of which involve the use of active elements (transistors, FETs, etc). The use of active elements allows the coherent summation of all the individual currents with no restriction on bandwidth, other than that set by the response of the individual active elements.
The elimination of the backward wave effects allows practically unlimited power handling capability together with potentially unlimited bandwidth. The present invention described hereinbelow also makes the job of matching the velocities of the optical and microwave wave easier, in that the velocity matching can be performed outside of the detector. This drastically reduces the constraints on the size and thickness of the photodetectors, thereby permitting device geometries that could not be realized with the conventional approach.
The traveling-wave device in accordance with the present invention represents a technological advance that will allow the achievement of higher operating currents at high modulation frequencies, a high desirable characteristic for systems requiring high dynamic range. Potential applications are in antenna remoting, spacecraft RF interconnects, analog and digital links, and any and all applications requiring a fiber optic link having a high-spur free dynamic range.
A preferred embodiment of the invention includes a plurality of discrete waveguide photodetectors. Each discrete waveguide photodetector has a maximum detectable modulation frequency, and each is serially interconnected with all of the others. Each discrete waveguide photodetector has a length less than a quarter of a microwave wavelength at the maximum detectable modulation frequency. Each discrete waveguide photodetector provides a respective discrete waveguide photodetector output current. Each respective discrete waveguide photodetector output current is coherently summed to provide an RF output current. Respective transmission lines are provided coupling each discrete waveguide photodetector to a summer. Lengths of the respective transmission lines are adjusted to provide for current waves traveling in the respective transmission lines to arrive at the summer in phase and sum constructively. The summer can include a plurality of active devices in a common emitter configuration, the base of an active device being coupled to a respective transmission line and to ground through a terminating load. The summer can alternatively include a plurality of active devices in a common base configuration, the emitter of an active device being coupled to a respective transmission line and to ground through a terminating load.